Phased array ultrasonic testing (PAUT) is an advanced method of ultrasonic testing (UT) that has applications in industrial nondestructive testing. Common applications are to find flaws in manufactured materials such as welds.
Single-element (non-phased array) probes, known technically as monolithic probes, emit a beam in a fixed direction. To test a large volume of material, a conventional probe must be physically moved or turned to sweep or scan the beam through the area of interest.
In contrast, the beam from a PAUT probe can be moved electronically, without moving the probe, and can be swept through a wide volume of material at high speed. The beam is controllable because a PAUT probe is made up of multiple small elements, each of which can be pulsed individually at a computer-calculated timing. The term phased refers to the timing, and the term array refers to the multiple elements. The elements of the probe that contribute to beam formation is defined as the aperture of the beam; the aperture can include a portion or all of the elements of the PAUT probe.
During typical inspections of welds, multiple beams are generated from a single or multiple apertures at various incidence angles. These generate an image showing reflections (or diffractions) of the ultrasonic waves that are associated with defects within the part over the scanned weld's areas of interest (where defects are expected to be found). For cases where the aperture is fixed and only angles are changed, the image is called a sectorial scan or s-scan.
In order to have an appropriate coverage of the weld area, it is almost always required to combine inspection from both sides of the weld. For defining the inspection, standards and normalized practice cover the guidelines for defining the probe and beam configuration, an example being the Nondestructive Examination (Section V) Boiler and Pressure Code of American Society of Mechanical Engineers. A weld inspection typically involves the use of a wedge, which defines a first mechanical incidence angle to generate an s-scan with shear waves in the 40 to 70 degree range of refraction angle, and a mechanical scan of the weld by moving the probe arrangement parallel to the weld axis.
A recurring problem associated with weld inspection with phased array ultrasonic scans is discrimination of relevant indications from acoustic reflections within the wedge, various ultrasonic paths involving mode conversion, or reflection from the weld geometry itself (referred to as geometric echoes).
FIG. 1a shows some typical problems with various positions that could be used for the inspection of a weld 104 in a flat part 102 using phased array probes 106a and 106b. Doing an NDI ultrasonic inspection, the inspector usually targets specific areas of the part 102 to be screened for defects. However, the sound has to travel a distance in the part 102 that does not need to be screened, for example the first leg of an ultrasonic beam 108.
Noise echoes from a flaw 112, for example, can become detrimental in the PAUT inspection. Most of the time these noise echoes will appear in areas that are not relevant for the weld inspection. To improve the signal to noise ratio (SNR), it is a common practice that inspectors define gates to select which section of the signal they want to keep. (Signals outside of the gates are discarded from the top/side view representation). Several types of gates can be placed to select a certain section along either the ultrasonic beam 108, a range of depth in the part, or, in the present disclosure, 2D polygons 120 and 122 on the weld cross section, which are referred to here as 2D gates.
One example of 2D gating is that gate 120 is used to screen the whole section of weld 104 to enable analysis of all signals coming from the weld, but discarding the noise echo from flaw 112. The 2D gate 122 used to screen only the fusion section of the weld to analyze an indication 118, but discarding noise echo from flaw 112. Another 2D gate can be used to screen only the toe crack section of the weld to enable analysis of a defect but discarding noise echo from flaw 112, or screen only the body of the weld to analyze porosity but discarding noise echo from flaw 112.
Continuing with FIG. 1a, 2D gates permit selection of a signal in relation to the real origin of the signals associated with ultrasonic beam 108 in part 102; this is the most effective gating process for improving the signal to noise ratio. Selecting that signal discards noise echo from flaw 112, and keeps the valid indication 118 for analysis. This type of gate can also be used to help identify defect types that are occurring in a particular section of weld 104. The main output of the 2D gates are top and/or side views using a color or grayscale palette, on which the noise signal is filtered to retain information relevant only to the inspection of 2D gates 120 and 122.
FIG. 1b is a schematic view of an example scan using a simplified representation of a manual scanner 404 linking the two phased array probes 106a and 106b. Each probe is placed on part 102 on each side of the weld 104. The scanner is then moved along the weld in a motion represented by a scan path 406 and a scan path 408. As can be seen, the paths are not perfectly straight, and they drift to the left and to the right causing mechanical drift from the weld.
Both the scan paths 406 and 408 are identical, but shifted along the weld depending on the probe position. It would be desirable for the probes to have coherent gating and tracking that automatically adjusts the position of the flaw indications in relation to the weld, so that information from multiple probes can be correlated.
Such concepts in FIGS. 1a and 1b, in particular the use of 2D gates, require good precision in positioning the probe relative to the weld, and keeping that relative distance constant over all the acquisition. This is difficult to achieve with an automated system, and even more so when doing a manual scan where drift is inevitable.
Part overlay is a known concept in many domains of existing practice in the field of NDT/NDI, including NDI data analysis, as it helps the inspector visualize the relative position of flaw indications in relation to the weld geometry. In order to conduct the analysis of the PAUT signals, it is typical current practice to manually adjust the probe-to-weld distance to fit echoes on a PAUT view that illustrate signals in the weld cross section, and which are related to the geometry of the part on the weld overlay One of the objectives of the present disclosure to automate this data analysis operation for all positions of the scan.
A solution for this problem has been found in automated girth weld inspection through use of the zone discrimination technique. The use of PAUT for inspection of pipeline girth weld has been described in various publications such as the American Society of Mechanical Engineers 2005 article “Pipeline Girth Weld Inspection using Ultrasonic Phased Arrays” (by Michael Moles, Noel Dubé, Simon Labbé, and Ed Ginzel), and incorporated in industrial standard practice such as the 2011 ASTM E-1961-11 publication “Standard Practice for Mechanized Ultrasonic Testing of Girth Welds Using Zonal Discrimination with Focused Search Units.” In practice, the technique involves a precise gating of the signals at specific zones where defects are expected. The downside of this method is the very high precision level required for positioning the probe relative to the weld, which makes the method suitable only for high precision automated inspection.
Another solution is to conduct a thorough analysis of the data acquired during the inspection by investigating each and every indication reported in the inspected part thickness range, by using depth related gates. While this method has been adapted for manual inspection, it is also time consuming and operator dependent.
Some automatic weld tracking methods are used in existing practice, such as U.S. Pat. No. 8,365,602, but they typically require direct access to the weld area. They require additional probes located on the top of the weld, and are typically suitable only for regular and predictable weld processes.
It would be therefore be desirable to have a method allowing precise gating of the ultrasonic signal for the inspection of a weld without the use of a precise positioning scanner.
It would also be desirable for this method to be as independent of the operator as possible.
It would also be desirable for this method to use only the existing or required hardware, with no extra dedicated tracking probe or other external device.
It would also be desirable for this method to be applicable to relatively harsh welding conditions.